Electric contacts



PaienteiiAug. 14, 1945 UNITED STATE PATENT. "orslcs ELECTRIC CONTACTS Eric I. Shobert, 11, St. Marys, 2a., summto Stackpole Carbon Company, St. Marys, 2a., a

corporation of Pennsylvania Application May 1a, 194:, eriallilmdflflt'i 5 Claims.

This invention relates to electric contacts, and more particularly to contact elements for heavy duty, low voltage aircraft and the like relays.

Contacts oi cadmium oxide and silver have been used for such purposes, but they have been subject to the disadvantage that they undergo excessive transfer of material, which may result in possible sticking and severe arcing after ;extended use, say after 50,000 operations, and if an overload occurs on a relay which has undergone severe transfer, the contacts may-not break the arc and open the circuit, which may, obviously,

this general type in uniting them, as by brazing or welding. withcontact arms, and various expediehts have been employed for providing a metallic face for that purpose. I have discovered, however, that a substantially pure metallic .face maybe provided easily and simply for such urposes by exposing one face of the compact to reducing conditions during the baking'operation.

' This may be, and preferably is, accomplished by supporting the compacts on a carbon surface during the baking operation. Inasmuch as bakdefeat the purpose of the relay and have serious 1 consequences. I v A primary object of this invention is to provide electric contact elements-composed of contact metal, most suitably silver, and cadmium oxide that are much more resistant to material transfer and consequently possess a longer and more useful life than similar contacts heretofore available commercially.

' A further object is to provide a method oi making such contacts which is simple, inexpensive and efficacious, and which in its preferred em-- bodiment results directly in contacts ready to be united to contact arms by brazing or welding.

ing is conducted in an, oxidizing atmosphere, the carbon and oxygen will react to form carbon monoxide which willln turn be converted to carbcn dioxide. However, carbon monoxide will be present at the interface formed by contact of the face 0! the compact and the carbon surface upon which it rests, and will reduce the cadmium oxide at that race and to a small depth inwardly of the contact, thus forming cadmium metal. By baking at a temperature above the boiling point of cadmium (1430 F), the cadmium formed by Other objects will appear from the following I now prefer that the mixture shall containfrom about 9 to 11 per cent by weisht oi cadmium oxide and the remainder silver powder.

The compacted bodies formed from such mixtures are baked at a temperature adapted to mium oxide (CdO) although for many purposes confer the mechanical strength and electricalproperties necessary in such an element. While that temperature may vary, in the preferred practice presently to be described the bodies are reduction of the oxide is volatilized, leaving a pure silver surface. Some of this vaporized cadmium may be deposited on the surfaces of the compacts which are i'reely exposed to the oxidiz- 1 lug atmosphere, where it will be reoxidized to cadmium oxide. In this manner therels formed a pure metallic face for connection to a contact arm, and in the preferred practice the face is. sumtantially pure silver. The depth to which the cadmium oxide will be removed will depend in part on the length of the baking step, but it need be sufllcient only to provide for adequate weldin to the contact arm.

Silver may be oxidized superficially at the other faces, which are freely exposed to the oxidizing atmosphere. This action is slight, however, and does not detrimentally aflect the properties of the contact. The silver oxide may be volatilized as it :iornis, but even if it does not the desired metallic silver contact surface" is provided easily by afinal sanding or other cleaning of'the contact.

, After the compacts have been repressed they are ready to be welded to the contact arm. Welding as referred to hereinand in the appended claims, includes, of course, brazing and related modes of attaching the contact to its arm to provide a connection that is mechanically strong and of satisfactory electrical properties.

A common practice is to silver plate the welded assemblies thus provided, the usual procedure being to bright dip them before the plating operation. Cadmium oxide is readily soluble in many acids,

but I have found that loss of cadmium oxide from the surface of the contact during bright dipping and silver plating may be avoided easily by applying a thin layer of paint, lacquer or similar coating composition to the surface of the contact prior to dipping and plating. The paint is removed readily from the contact surface after lating by means of a suitable solvent. The contacts may then be burnished and sanded lightly to remove any film of solvent or coating composition that may remain.

As an example of the practice of the invention, a mixture of per cent by weight of cadmium oxide and 90 per cent by weight of silver powder was mixed in a ball mill, and pressed into contact element shapes under a pressure of 25 tons per square inch. The compacts were baked on a carbon plate in air at 1500 F. for five minutes. After cooling they were repressed and then welded to copper supporting arms. The assembled elements were then tested on a Guardian 24-vdlt, ZOO-ampere aviation type intermittent duty relay. In this life test the contacts exhibited substantially no transfer of material in comparison with other commercially available silver-cadmium oxide contacts, which showed considerable transfer.

The pressure used in repressing after baking will depend upon various factors but generally speaking this may be done at about 20 to 60 or 70 tons per square inch, and tons per square inch is desirable for many purposes.

Although the inventionhas been described with particular reference to silver-cadmium oxide contacts, other contact metals may be used. For example, copper behaves in a manner similar to silver when mixed with cadmium oxide, but in making contacts from copper the baking should desirably be conducted in a neutral or reducing atmosphere, and cadmium oxide removed by acid from the face of the element which is to be connected to the contact arm, the other faces being protected against the solvent action of the acid.

According to the provisions of the patent stat- 'utes, I have explained the principle and mode of practicing my invention, and have described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

I claim:

1. That method of making a silver-cadmium oxide electric contact element which comprises pressing an intimate mixture of silver powder and powdered cadmium oxide/containing from about 0.1 to about 20 per cent by weight of said oxide to compact it, heating the compact in an oxidizing atmosphere while exposing a single face to reducing conditions and thereby reducing the cadmium oxide at that face.

2. A method according to claim 1, said mixture containing about 9 to 11 per cent of cadmium oxide.

3. That method of making a silver-cadmium oxide electric contact element which comprises pressing an intimate mixture of silver powder and powdered cadmium oxide containing from about 0.1 to about 20 per cent by weight of said oxide to form a compact, supporting a single face of said compact in contact with carbon and heating in an oxidizing atmosphere and thereby reducing cadmium oxide at said face.

4. That method of making a silver-cadmium oxide electric contact element which comprises pressing an intimate mixture of silver powder and powdered cadmium oxide containing from about 0.1 to about 20 per cent by weight of said oxide to form a compact, supporting a single face of said compact in contact with carbon and heating in an oxidizing atmosphere at a temperature above the boiling point of cadmium and below the melting point of silver and thereby reducing cadmium oxide at said face and volatilizing the resultant cadmium therefrom to leave a silver surface substantially free from cadmium oxide, then repressing the body. v

5. That method of making a silver-cadmium oxide electric contact element which comprises pressing an intimate mixture of silver powder and powdered cadmium oxide containing from about 0.1 to about 20 per cent by weight of said oxide to form the contact body, supporting a single face of said body in contact with carbon and heating in air at a temperature above the boiling point of cadmium and below the melting point of silver and thereby reducing cadmium oxide at said face and volatilizing the resultant cadmium therefrom to leave a silver surface substantially free from cadmium oxide.

' ERLE I. SHOBERT, II. 

